1. Field of the Invention
The present invention relates to a fuel supply control device of an internal-combustion engine and, more particularly, to a step motor drive control for controlling a step motor which drives a control means such as a throttle valve for controlling the quantity of air or the quantity of fuel to be supplied to an internal-combustion engine such as a gasoline engine or a diesel engine.
2. Description of the Related Art
This type of step motor drive control is required to have high accuracy and a fast response time.
For example, when the throttle valve is driven to control its opening by the step motor, the step motor must be operated frequently to turn the throttle valve from the open side to the closed side, or reversely from the closed side to the open side, and from drive to stop, or to switch a drive control mode, in accordance with a demand from a drive control system, in quick response to the demand to control the amount of opening of the throttle valve.
The step motor in this case is likely to go out of phase owing to the inertia force of the throttle valve. To prevent this out of phase condition, various methods have been developed such as the provision of a large margin to a required torque of the step motor relative to a load, and, particularly, the hold of phase excitation immediately before the input of an OPP signal at the time of an opposite turn for a specific period of time (Japanese Patent Laid-Open No. Sho 58-204799), and the delay of supply of a drive pulse signal for a specific period of time (Japanese Patent Laid-Open No. Sho 62-294743).
The out-of-phase condition occur during other than the opposite turn too, and it is difficult to completely prevent the out of phase condition by the above-described methods even during the opposite turn.
Furthermore, increasing a torque margin of the motor is accompanied by such problems as the use of a large-sized motor and the occurrence of an overheating resulting from the increased consumption of electric current.
Furthermore, with an instantaneous disconnection of a wiring harness or a coupler taken into consideration, the step motor will lose synchronism when driving with a sufficient torque, resulting in a failure in the control of throttle valve opening which requires high accuracy.
Besides the above-mentioned methods, there is a method for controlling a motor position by feeding back a value detected by a sensor which detects the position, or opening, of the throttle valve. This position sensor, however, is required to be of high accuracy and high reliability, and moreover sacrifices such an advantage as the possibility of open-loop control which is a feature of the step motor.